In recent years, an inverter device has been used in fields of household electric articles, industrial electric power apparatuses, and the like. An inverter device using commercial power source (alternating current source) includes a converter part which implements forward conversion from alternating current source into direct current, a smoothing circuit part, and an inverter part which implements reverse conversion from direct current voltage into alternating current. A main power element of the inverter part is chiefly constituted by an insulating gate-type bipolar transistor (hereinafter sometimes referred to as an IGBT (Insulated Gate Bipolar Transistor)) capable of achieving high-speed switching.
According to an inverter device for power control, a current rating and a voltage rating per one chip of a transistor range approximately from several amperes to several hundred amperes, and approximately from several hundred volts to several thousand volts, respectively. Accordingly, in the case of a circuit operated by successive changes of the gate voltage of an IGBT by using resistance loads, power corresponding to a product of current and voltage is generated as heat inside the IGBT. This condition produces necessity for equipment of a large radiator, and deteriorates conversion efficiency of power. Moreover, an increase in temperature of the transistor itself causes thermal destruction depending on a combination of operating voltage and operating current. For this reason, a resistance load circuit is not generally used.
The load of an inverter device is provided by an electric induction motor (inductive load motor) in many cases. Therefore, an IGBT generally operates as a switch, and controls power energy by repeating ON-state and OFF-state. In switching an inverter circuit using an inductive load, possible conditions are a turn-off process from ON-state to OFF-state of a transistor, a turn-on process from OFF-state to ON-state of a transistor, and an ON-state of a transistor.
The inductive load connects with an intermediate potential point between upper and lower arms, wherefore a direction of current flow in the inductive load is both positive direction and negative direction. The current flowing in the load is returned from the load connection end toward the high-potential power source side, or drawn toward the ground side. Accordingly, equipment of a free wheel diode is needed for the purpose of circulation of large current flowing in the inductive load between the load and the closed circuits of arms. FIG. 6 shows an inverter circuit (full-bridge circuit) using a conventional IGBT and a free wheel diode. In the case of a small-capacity inverter device, an MOSFET (Metal Oxide Silicon Field Effect Transistor) is used instead of an IGBT in some cases.
A trench gate type IGBT (see Patent Document 1), a carrier accumulating-type trench gate IGBT, and others are proposed as a structure lowering ON-state voltage of an IGBT. In addition, a reverse-conducting-type IGBT (RC-IGBT) containing the function of a free-wheel diode in one chip is proposed (see Non Patent Documents 1 and 2).